This invention relates to novel unitary composite laminate structures useful in the manufacture of products such as railroad cars, truck trailers, shipping containers, buildings, and to methods for making such unitary structures and products.
Multi-layer panels in which synthetic materials are adhesively bonded one to the other are known. These panels are interconnected by adhesive bonding, with adhesive curing occurring at ambient temperature and pressure. Accordingly, these panels have significant limitations in structural applications because of their propensity to delaminate under load conditions requiring a high level of tensile and shear strength.
The following are examples of such adhesively interconnected panels. In U.S. No. 4,101,704, a low energy absorbent panel comprising a first inner layer of elastomeric non-cellular polyurethane having a low compression set, is interconnected to a less compressible second inner layer comprising a flexible polyurethane matrix in which a multiplicity of hollow bodies are embedded. The first and second inner layers are encased within a pair of outer skin layers. U.S. No. 4,361,613 describes an insulating, fire-resistant, multi-layered panel comprising first and second spaced apart outer sheets overlying upper and lower composite facing sheets interconnected to a foam core. The outer sheets are made from a polyvinyl fluoride material, Tedlar.TM.. A layer of chopped fiber strands and adhesive resin are built-up into the upper and lower composite facing sheets. U.S. No. 4,617,217 provides a beam of great length in the form of a box structure comprising a core of a hard synthetic resin foam, such as polyurethane foam, around which are wound at least two superimposed independent layers of glass fabric whose threads are oriented at 45.degree. with respect to the longitudinal axis of the core. These fabrics are impregnated with a resin which is polymerizable. U.S. No. 3,535,198 is a laminated multi-layer structural panel for outdoor use as wall or roof panels formed of a pair of halflaminates comprising a foamed core of polyurethane or a cyclic ether bonded in situ to a pair of rigid outer facing sheets. The half-laminates are bonded together along their foam interface with an adhesive material.
Attempts have been made to incorporate multilayer, interconnecting panels in structural applications such as railroad cars, trailers and buildings. Shear stress and tensile-elongation problems are present when these structures are employed for such uses. Furthermore, none of these are unitary composite structures. For instance, U.S. No. 2,962,323 relates to an insulating chamber which must be supported within a fixed structural enclosure for the above uses. Since these panels are not effective as load-bearing structures, an inner structural enclosure must be provided. U.S. 3,989,562 relates to a container or a body for a road vehicle or rail vehicle, the body comprising an inner tube formed of a wound laminate of synthetic resin impregnated material, an outer tube formed of a wound laminate of synthetic resin impregnated material and spaced from the inner tube, and insulating material and reinforcing material all being securely interconnected. The impregnated material may comprise filaments and/or fabric. The insulating material may be a foamed material such as rigid lightweight polyvinyl chloride. There is no substantial interconnection between the respective windings thereby creating major delamination problems when structural shear forces are applied. More specifically, these delamination problems will result between layers of the windings. The structure produced is heavy and extremely brittle. There are also major expansion-contraction coefficient problems between the construction materials, i.e. between the windings and the metal. U.S. No. 4,082,882 relates to multi-layer structural sandwich members comprising an inner core of plywood, medial layers of fiberglass mats of crossed fiber or woven roving with a waffle pattern and a general purpose resin, and outer layers of films of an acrylate copolymer. Adhesion problems result when plywood is bonded to a polymeric at room temperature. Plywood is also a problem when used as a core material since it is organic and therefore biodegradable, and also has a high moisture content. U.S. No. 3,573,144 is directed to a sandwich panel comprising first and second face sheets and a rigid core bonded therebetween. The core comprises parallel rows of alternately oppositely undulated resin impregnated fibrous webs and a series of spacer blocks extending transversely of the fibrous web rows and alternately under and over adjacent webs, to define the oppositely undulated arrangement of the web. The spacer blocks can comprise polyvinyl chloride foam and the fibrous webs can comprise glass cloth. The top and bottom face sheets are unitary sheets of metal or composites such as glass-resin composites especially glass cloth with epoxy resin impregnation. The resin impregnated fibrous webs of the core structure also employ epoxy resin throughout the glass cloth to rigidly fix the blocks in their arranged locations. Shear strength is a substantial problem in view of difficulty in bonding together disparite core elements as described above. Finally, U.S. No. 3,003,810 is a vehicle body constructed of a composite structure of synthetic resin materials. The trailer body comprises assembled modular panel units. A typical panel consists of a central core which is adhesively interconnected to a rigid sublamina. The exposed face of each sublamine is covered with a structural skin. The core is made of a lightweight, low density, cellular material. Sublaminae is a high density, foamed polymer or plywood or hardboard. Facing skins are resinous materials which can be reinforced with a fibrous material, such as a woven filamentary glass web.
Therefore, a need exists for a laminate structure of unitary composite design for use in previously described products which is bonded together to avoid delamination under shear conditions, and which exhibits a high degree of tensile elongation strength under high load bearing conditions.